In fuel cell-supported transportation systems, so-called chemical reformers are used for obtaining the required hydrogen from hydrocarbon-containing fuels or for providing the reaction temperature of catalytic burners or afterburning devices.
All the substances needed by the reformer for the course of reaction such as air, water and fuel are ideally supplied to the reformer in the gaseous state. However, since the fuels such as methanol or gasoline, and water are preferably stored onboard the transportation system in liquid form, they must be heated so as to be vaporized shortly before being fed into the reformer. This requires a pre-evaporator (separate or integrated into the reformer) capable of providing adequate quantities of gaseous fuel and water vapor.
Since the hydrogen is normally consumed immediately, chemical reformers must be capable of adjusting the production of hydrogen to the demand without delay, e.g., in response to load changes or during start phases. Especially in the cold start phase, additional measures must be taken, since the reformer does not provide any waste heat. Conventional evaporators are not capable of generating adequate quantities of gaseous reactants without delay.
It is therefore practical to introduce the fuel into the reformer in a finely divided form with the aid of an atomization device, in which case, provided that there is a sufficient supply of heat, the vaporization process is improved by the large surface area of the finely divided fuel.
So-called catalytic burners provide the temperature required for the chemical reaction, in which the fuel among other things is reformed to hydrogen, for example. Catalytic burners are components featuring surfaces coated with a catalyst. In these catalytic burners, the fuel/air mixture is converted into heat and exhaust gases, the generated heat being conducted to the suitable components such as the chemical reformer or an evaporator via, for example, the lateral surfaces and/or via the warm exhaust-gas stream.
The conversion of fuel into heat is highly dependent on the size of the fuel droplets striking the catalytic layer. The smaller the size of the droplets and the more uniformly the catalytic layer is wetted with the fuel droplets, the more completely the fuel is converted into heat and the higher is the efficiency. In this way, the fuel is also converted more quickly, reducing pollutant emissions. Fuel droplets that are too large in size result in a coating of the catalytic layer and hence in a slow conversion rate. This leads to poor efficiency, especially in the cold start phase.
In addition, such an atomization system may be used for metering a urea-water solution directly into the exhaust-gas stream for exhaust-gas aftertreatment.
Devices for reforming fuels are described in, for example, U.S. Pat. No. 3,971,847. According to this document, metering devices located relatively far away from the reformer are used to meter the fuel via long supply lines into a temperature-adjusted substance stream and disperse it via a metering aperture at the end of the supply line into the substance stream, which flows to the location of the actual reforming process.
A particularly disadvantageous feature in the conventional devices described in the above-mentioned document is the fact that the long supply lines result in delays and inaccuracies in fuel metering, especially in the case of sharp load changes or warm start phases. If fuel metering is resumed following a stop phase for example, while the fuel is evaporating under the temperature influence from the supply line, this results in a delayed metering of fuel into the temperature-adjusted substance stream and to the reforming process, because the dead-space volume in the supply line must first be replenished. The same problem arises in the case of a particularly small load. Furthermore, long supply lines stand in the way of compact construction while increasing proneness to error and assembly cost.